The present invention generally relates to a writing instrument that includes a means for calculating the Body Mass Index (BMI) of a person. In general, the Body Mass Index (BMI) of a person is calculated based on the weight and height of the person. For example, a BMI may be determined based on the following equation: M/L2, where M is the body weight in kilograms, and L is the body height in meters. It is recognized that other equations may be used for calculating the BMI. For instance, the body weight may be expressed in pounds, and the body height by inches. 
The BMI is a useful indicator of a person's body composition and can be used in various applications. For example, health-care providers can use a BMI to determine the proper amount of medication to prescribe for a patient. The BMI of a patient is used often in prescribing medication for treating diabetes and other illnesses.
Wall charts or reference books containing a BMI scale for patients are commonly used in the health care industry. The BMI chart can be hung on the wall of an examination room so that a health-care provider may refer to the chart. Also, various electronic devices are known for calculating the BMI of a patient. These devices have electronic input sensors for entering personal body data, electronic means for calculating the BMI based on the entered body data, and a graphics window for electronically displaying the BMI and other related information.
In a routine physical examination of a patient, a health-care provider must handle and use a large number of separate products including, for example, various medical instruments, pieces of diagnostic equipment, stethoscopes, height/weight scales, writing pens, patient charts, prescription pads, and the like. Although conventional electronic devices may be effective in calculating a patient's BMI, the health-care provider must handle and operate these devices separately in addition to various other items. Many health-care providers would like to reduce the number of items that they must handle during the examination of a patient.
U.S. Pat. No. 5,028,764 to Kuhlengel discloses a pen with rotatable cylindrical elements that is used to perform a simple calculation in the multiplication of two numbers. U.S. Pat. No. 6,104,603  to Wang discloses a writing pen with an integral electronic calculator including printed circuit board, keyboard and power supply. In each of these patents, the invention provides its user no capability to perform a specific non-linear calculation, as the former will not permit a non-linear calculation and the latter relies upon the user to supply a formula, in addition to its dependence on electronic, rather than manual components. Therefore, neither patent involves a device which may be manually employed to quickly determine the numerical outcome of a specific non-linear relationship equation, such as the Body Mass Index of a person, based jointly on multiple variables, in this case the two factors being their height and weight.